The object of the present invention is to provide a stabilised high vacuum drying installation for industrial hides and similar products. Drying installations of this kind are normally used in tanning industry to eliminate water and residual moisture from hides after tanning treatment and before finishing processes.
Drying installations of the above type are known for a long time and include, in accordance with the preamble of claim 1, a plurality of drying beds for the products to be dried, including essentially a heating platform on which the hides are laid and which are closed by hermetic covers, vacuum means connected to the drying beds to aspirate vapours coming from the hides and reduce the pressure and the equilibrium temperature to a lower working value, and a circuit with valve means to selectively connect the drying beds to the vacuum means.
The vacuum means generally include one or more vacuum pumps, for example of the liquid ring type, a compressor, a blower or an ejector. Such vacuum devices can be placed in the aspiration circuit so as to work separately or in combination.
From the European patent No. 0 689 613 in the name of the same applicant, a high vacuum installation of the above mentioned type for industrial hides dryers is known, wherein the vacuum means include a vacuum pump for reducing progressively the pressure to a first predefined value corresponding to a first working temperature, and a secondary aspiration device is connected upstream of the main vacuum pump and working in series with the latter to further reduce the pressure so as to allow to increase instantaneously evaporation and/or to reduce the equilibrium temperature to a second value lower than the first one. With such installation, which is commonly considered overcharged or xe2x80x9cturbochargedxe2x80x9d, it is possible to reduce drastically hide drying time while maintaining vapours equilibrium temperature within comparatively low values.
Alternatively, it will be possible to reduce noticeably the evaporation temperature by reducing drastically hide thermal stress and thus improving remarkably their final quality.
From EP-A-0 689 659, in the name of the same applicant, a vacuum installation for industrial dryers is known wherein the aspiration circuit provides valve means to connect every platform or drying bed, initially at atmospheric pressure, to the secondary aspiration device until a degree of vacuum close to the steady state is reached in that bed, and subsequently with the main vacuum pump so as to eliminate oscillations or pressure swingings in the remaining drying beds, which are under vacuum.
In practise, stabilised high vacuum installations are provided, in which both solutions offered by the two above-cited patents are present in order to attain both advantages, i.e. low evaporation temperature and pressure stabilisation in the drying beds.
However, such installation practically involves the use of at least three pumps, a main vacuum pump for the extraction of the vapours from the circuit, a secondary vacuum pump to reduce pressure in a drying bed in turn and an aspiration device like a compressor, blower or ejector to reduce further the pressure to the minimum working value.
A primary object of the present invention is to eliminate the drawbacks described above by providing an installation for high vacuum industrial hide dryers with characteristics of high efficiency and relatively simple structure.
A particular object is to provide a vacuum installation of the above kind with a lower number of aspiration devices so as to reduce energy consumption and increase installation reliability.
In accordance with one preferred aspect of the present invention, there is provided a stabilised high vacuum drying installation for industrial hides and similar products comprising a plurality of drying beds comprising heating platforms and hermetic covers, vacuum means connected to the drying beds to extract vapours coming from the hides and reduce the pressure to minimum working value, a circuit with valve means to selectively connect said drying beds to said vacuum means, characterized by the fact that said valve means comprise, downstream of each drying bed, at least one cutoff valve for selectively connecting said bed, initially at atmospheric pressure, to the vacuum means, and at least one check valve for automatically isolating the remaining bed already at working pressure so as to avoid pressure oscillations internally thereof.
Thanks to the insertion of check valves downstream of the drying bed, it will be possible to eliminate one vacuum pump normally present in stabilised high vacuum installations to reduce the pressure, with consequent reduction of the complexity and operating costs of the installation.